1. Field of the Invention
This invention relates to illumination systems. In particular the invention relates to illumination systems for producing a beam of light for illuminating a spatial light modulator, the spatial light modulator producing a spatially modulated beam of light which may be projected onto a display screen.
2. Discussion of the Background
The spatial light modulator may take the form, for example, of a digital micromirror device also known as a deformable or deflectable mirror device (DMD). Digital micromirror devices comprise an array of deflectable mirror elements, each mirror element being mounted on a torsion element over a control electrode. Applying an electric field between each mirror element and the associated control electrode causes the mirror element to pivot. Thus the direction of light reflected from each mirror element may be changed by application of suitable electrical address signals to the digital micromirror device, the electrical address signals usually being derived from an input video signal. In particular, each mirror element may be caused to reflect light either in an xe2x80x9conxe2x80x9d direction towards a projector lens for projection onto a display screen, or in an xe2x80x9coffxe2x80x9d direction towards a beam dump. It is thus possible to spatially modulate a beam of light directed onto the array of mirror elements, the beam being projected onto a display screen so as to produce a projected image. The pixels of the image displayed on the display screen will be derived from one or more of the mirror elements of the digital micromirror device.
In order to provide a sufficiently intense light beam to address such a digital micromirror device, an arc lamp arranged to provide a substantially parallel output beam has been used as the light source. One example of such an arc lamp is described in our co-pending European Patent Application EP-A-0646284.
As a projection apparatus incorporating a pixellated spatial light modulator such as a digital micromirror device requires very uniform illumination across the array of deflectable mirror elements, it is known to incorporate an integrator rod, also known as a light pipe, in the light path between the light source and the array.
The operation of an integrator rod for producing a uniform beam for illuminating a spatial light modulator such as a digital micromirror device is illustrated schematically in FIG. 1.
In FIG. 1, a light source, for example the arc produced by an arc lamp is represented by the triangle labelled 1. A condenser lens 3 is effective to form an inverted image of the light source 1 onto the input surface 5 of a glass integrator rod 7 of a rectangular cross section.
Light entering the rod 7 will propagate through the rod by means of multiple reflections from the internal surfaces of the rod 7. The number of reflections which the light inside the rod 7 undergoes will depend on the angle of incidence of the light on the input surface 5 of the rod 7 and the length of the rod. An even number of internal reflections of the input light inside the integrator rod 7 will produce an inverted image of the light source 1 in the plane of the input surface 5. An odd number of internal reflections of the input light inside the integrator rod 7 will produce a non-inverted image of the light source 1 in the plane of the input surface 5. Thus a large number of both even and odd reflections will lead to multiple images of the source in the plane of the input surface 5 of integrator rod 7, where the orientation of each image is determined by the number of reflections. This effectively transforms the original non-uniform distribution of light at the input surface 5 of integrator rod 7 into a more uniform light distribution at the output surface 9 of the integrator rod 7.
The integrator rod 7 will also be effective to create an output beam of a cross-sectional aspect ratio matched to the output face of the integrator rod 7. This is particularly beneficial in a projection apparatus incorporating a digital micromirror device as the circular cross-section beam produced by, for example, an arc lamp will be converted into a rectangular cross-section beam which may be designed to match the aspect ratio of the digital micromirror device.
A more detailed explanation of the use of an integrator rod in an illumination system is given in xe2x80x9cModern Optical Engineeringxe2x80x9d by Warren J Smith, published by McGraw-Hill Book Company, 1990; pages 263 to 265.
Whilst the use of an integrator rod is particularly beneficial in a projection apparatus incorporating a digital micromirror device, the introduction of the integrator rod into the light path between the light source and the digital micromirror device may itself create optical aberrations in the beam which illuminates the digital micromirror device. In particular, flicker in the light beam produced in the arc lamp 1 caused by turbulent movement of the gas within the arc lamp may be focused close to the output surface 9 of the integrator rod 7, and appear in the projected image.
Furthermore, any dust appearing on the output face of the integrator rod will be focused by the projector lens on to the display screen.
It is an object of the present invention to provide an illumination system suitable for illuminating a spatial light modulator wherein the above disadvantages may be avoided, together with an optical component for use in such a system.
It is in some circumstances necessary to use two separate light sources to address a spatial light modulator. EP-A-0704737 discloses an illumination system for a deformable mirror device in which two separate light sources are used to illuminate a digital micromirror device. This enables light of two different colours to be alternately directed to the digital micromirror device. However, such an arrangement will suffer the disadvantage that any spatial irregularities in the beam produced by either of the two individual light sources will be projected directly onto the digital micromirror device.
It is a further object of the present invention to provide an illumination system for a spatial light modulator which may include more than one light source.
According to a first aspect of the present invention there is provide an illumination system for a spatial light modulator including a light source, means for imaging an image of the light source onto the input surface of an integrator rod, and a field lens interposed between the imaging means and the input face of the integrator rod, the field lens being effective to focus an image of turbulent light within the light source away from the output face of the integrator rod.
According to a second aspect of the present invention there is provided an illumination system for a spatial light modulator including an integrator rod interposed in the light path between a light source and the spatial light modulator, a protective transmissive layer being positioned at the output surface of the integrator rod.
According to a third aspect of the present invention there is provided an illumination system for a spatial light modulator including at least two light sources, an integrator rod, and means in respect of each light source effective to couple light from the respective light source into the integrator rod.
According to a fourth aspect of the present invention there is provided an illumination system for a spatial light modulator wherein at least one of the optical components is formed with a diffractive surface effective to cause light within unwanted wavelength bands to be removed from the light incident on the spatial light modulator.
According to a fifth aspect of the present invention there is provided a projection device for use in a projection system including an illumination system in accordance with any one or any combination of the first four aspects of the invention.
According to a sixth aspect of the present invention there is provided an optical component including an integrator rod modified for use in an illumination system in accordance with any one or any combination of the first four aspects of the invention.